This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

## Wednesday, November 07, 2007

### Singapore Chinese Girls School PSLE Math Prelim 2007 Question

When Mrs Lee was 40 years old, her son was twice her daughter’s age. Mrs Lee will be twice her son’s age when her daughter is 28 years old. How old will Mrs Lee be when her daughter is 20 years old?

When Mrs Lee was 40 years old, son was twice daughter’s age.

Model 1 (click on image below for clearer view)

When daughter turns 28 years old, Mrs Lee will be twice son’s age.

Model 2 (click on image below for clearer view)

Now that we know

- Mrs Lee is 28 years plus 1 unit older than son
- Son is 1 unit older than daughter

we can draw the next model when Mrs Lee was 40 years old again.

Model 3 (click on image for clearer view)

From Model 3 we can work out

(Mrs Lee) 3 units + 28 yrs ---- 40 yrs

3 units ----- 40 years - 28 years
3 units ----- 12 years

1 unit ---- 12 years divided by 3
1 unit ---- 4 years (the all important 1 unit)

Finally, we can work out Mrs Lee's age when her daughter turns 20 years old.

Model 4 (click on image for clearer view)

When daughter turns 20 years old, Mrs Lee will be -

20 years (daughter’s age) + 4 years (gap between daughter and son) + 28 years + 4 years (gap between Mrs Lee and son)

20 years + 4 years + 28 years + 4 years = 56 years.

Answer: Mrs Lee will be 56 years old when her daughter turns 20 years old.